The invention relates to a method of preparing phthalide by the catalytic dehalogenation of 3-chlorophthalide in the presence of HCl acceptors.
Numerous processes are described in the literature for the preparation of phthalide by reactions of o-disubstituted benzenes, such as phthalic acid dichloride, o-xylyl chloride, o-tolylic acid, o-phthalic aldehyde, o-xylylene glycol, phthalimide and the like. Phthalide is formed from these starting substances only after a plurality of difficult process steps or in an unsatisfactory yield. Other methods of preparing phthalide are the reductions of phthalic acid anhydride, which are performed, for example, with zinc and glacial acetic acid (Ber. 17, 2178 [1884]), by catalytic hydrogenation with noble metal catalysts (F. Zymalkowski, "Katalytische Hydrierung im Organisch-Chemischen Laboratorium," Ferd. Enke Verlag Stuttgart, 1965, page 186), or in the presence of nickel (German Reich Pat. No. 368,414).
In these reductions of phthalic acid anhydride to phthalide, reaction water forms, which leads to trouble in the course of the hydrogenation and results in contaminated products. These disadvantages are partially overcome in the electrochemical reduction of phthalic acid anhydride (Bull. Chem. Soc. Japan 7, 127 [1932], German Offenlegungsschrift No. 2,144,419), yet the performance of electrochemical processes generally presents technical difficulties and involves a relatively great investment in apparatus.